should I just get the 4GB of RAM then put either the 16 or 32 in myself to save.

I believe that only the 27" iMac has user serviceable RAM through small door in the rear, and the base spec machine has 8 BG. If you want a smaller machine I believe the ram you buy is what you're stuck with.

The latest version 21.5" iMac does not have user-serviceable RAM, and I believe that it only has half the capacity of its 27" sibling. If you custom-order a 21.5" iMac from Apple, the upgrade from the standard 8 GB of RAM to 16 GB of RAM costs $200.

On a 27" iMac, you could pop in a third-party 8 GB module to get 16 GB total for around $50, and you would have the option of expanding the machine to 24 GB or 32 GB at your leisure.

Older 21.5" iMacs have four user-accessible memory slots, and can take 16 or 32 GB of RAM. They also have optical drives and Firewire 800 ports, but not USB 3.0 ones.